1. Field of the Invention
The invention relates to the field of so-called FDR measurement (frequency domain reflectometry) and relates in particular to a method and an apparatus for phase calculation from attenuation values by means of a Hilbert transform for FDR measurements.
2. Description of the Related Art
FDR measurements are carried out using network analyzers, which can determine the magnitude and phase of the reflection factor, or by means of spectrum analyzers, which can measure only the magnitude of the reflection factor. In present-day measurement methods, the phase is calculated by means of a Hilbert transform using spectrum analyzers with a tracking generator. One important factor for the numerical quality of the Hilbert transform is the fact that all the pole points in the logarithmic attenuation function are located as far as possible to the left of the imaginary axis in the pole null point diagrams. In currently existing measurement systems, this is achieved by means of fully symmetrical parallel branching in the measurement circuit, which is produced by the use of a coupling element (coupler). However, couplers such as these have a very high intrinsic reflection factor with respect to the measurement object and a low insertion loss from the source to the measurement object and back again from the measurement object to the power meter. This circuit ensures only the preconditions for carrying out the Hilbert transform, that is to say all the poles in the attenuation function are located in the left-hand complex half-plane. Nevertheless, undesirable and relatively large errors generally occur in the calculation of the phase.